Well for one, people with serious mental illness often do drugs. Lots of drugs. And not just marijuana. Marijuana is actually pretty low down on the list. In other words, people with schizophrenia and other serious mental health problems are at high risk of addiction. And this will necessarily confound their study.

The authors assert that this is not the problem, that is, mental health susceptibility is not the cause of the marijuana use but rather, marijuana use is increasing the probability of psychosis. Their reasoning is that the cohorts excluded populations that already had a diagnosis of schizophrenia or other psychotic disorder, so they are studying people who showed no signs of mental illness, then became mentally ill. This is well and good, and absolutely the right thing to do, otherwise you would immediately see a huge effect simply because people with serious mental health problems are at high risk of addiction. But, is the assumption still valid?

Well, no. And my answer lies in smoking. Yes, plain old cigarettes. Do you have any idea how much schizophrenics smoke? Tons. It's unbelievable. In fact, approximately 90% of schizophrenics smoke, and of schizophrenics who smoke, 90% of them started smoking before onset of their illness. Further, evidence of a dose response effect, 68% of schizophrenics are heavy smokers, compared to 11% of the smoking population.

Now this is kind of astounding. Why don't we say that cigarettes are putting people at risk for psychosis? Just because something follows an event, doesn't mean that event is the cause. The population is choosing the drug, the drug is not creating the population. If you were to do similar study with cigarettes, or booze, you'd find a similar correlation (RR=1.94), and dose response etc (although some articles have suggested a protective effect as well - albeit in older cohorts). Eliminating the cohort that already had the diagnosis would not eliminate the effect, because schizophrenics overwhelmingly start smoking, and other drugs, before the psychotic break that defines the diagnosis.

Similar findings are often shown with alcoholism and early drinking, but suffer from the same assumptions. There are people with a tendency towards addiction, for various reasons, and at earlier ages they engage in drug-seeking behaviors. The mistake is thinking that when they become alcoholic or drug dependent in early life that the drugs caused the behavior, rather than considering the possibility that such behavior existed even before exposure to the drugs. Hell, I knew people growing up that would seek a high from every chemical at hand (long before they had even had access to pot or booze), not because they were addicted to those chemicals, or had even tried them before, but because they were the type that actively seek a high.

Correlation is not causation people. And this particular correlation is a hairy one, due to the tendency towards drug abuse that is characteristic of those suffering from psychotic disorders. I would put little trust in this particular study's conclusions as the more likely phenomenon being studied is the predilection for drug-seeking in adolescents who subsequently become schizophrenic.

The evidence is consistent with the view that cannabis increases risk of psychotic outcomes independently of confounding and transient intoxication effects, although evidence for affective outcomes is less strong. The uncertainty about whether cannabis causes psychosis is unlikely to be resolved by further longitudinal studies such as those reviewed here. However, we conclude that there is now sufficient evidence to warn young people that using cannabis could increase their risk of developing a psychotic illness later in life.